The present invention provides, among other things, methods of treating Argininosuccinate Synthetase Deficiency (ASD), including administering to a subject in need of treatment a composition comprising an mRNA encoding argininosuccinate synthetase (ASS1) at an effective dose and an administration interval such that at least one symptom or feature of ASD is reduced in intensity, severity, or frequency or has delayed in onset. In some embodiments, the mRNA is encapsulated in a liposome comprising one or more cationic lipids, one or more non-cationic lipids, one or more cholesterol-based lipids and one or more PEG-modified lipids
